The present invention relates to method and apparatus for heat sterilization of sealed ampules filled with medical fluid and moving along a slot formed in the upper wall of a microwave irradiation furnace communicating with a rectangular waveguide.
The sealed ampule filled with medical fluid such as an injectable medical fluid is usually subjected to a sterilizing treatment during its manufacturing process and microwave sterilization is well known as one example of said sterilizing treatment.
Such microwave sterilization relies on the phenomenon that medical fluid filling the ampule is heated upon absorption of the microwave energy and this is certainly advantageous in that desired sterilization is achieved in a very short time. However, such well known method of microwave sterilization is disadvantageous in that temperatures within the ampule are apt to become uneven, a trend of temperature rise (temperature rise curve) depends upon a particular type of medical fluid filling the ampule and a gaseous phase within the ampule is apt to be insufficiently heated, resulting in imperfect sterilization.
These Countermeasures are disclosed, for example, by Japanese Disclosure Gazette No. 1973-61609 entitled "Apparatus for sterilization of sealed ampule filled with medical fluid"; Japanese Disclosure Gazette No. 1973-59976 entitled "Apparatus for sterilization of medical fluid within ampule; and Japanese Disclosure Gazette No. 1979-34590 entitled "Method for sterilization within container".
The microwave absorptivity generally depends upon various factors such as the conductivity of the medical fluid and, in addition, the size of the ampule as well as the quantity of medical fluid filling the ampule. Therefore, microwave irradiation under the same conditions, without proper consideration of these factors, might disadvantageously lead to different degrees of temperature rise, depending upon the type of medical fluid and the size of the ampule.
In view of the above possible variables, none of the well known techniques as set forth above has effective adaptability for the temperature rise characteristic, i.e. the temperature rise depending upon the type of medical fluid, and thus none of these techniques achieve uniform sterilization.
More specifically, the apparatus in accordance with the above-mentioned Japanese Disclosure Gazette No. 1973-61609 aims at a uniform temperature rise within the ampule by rotating the ampule which is being held at an appropriate angle with respect to the vertical as the ampule is irradiated with the microwaves. However, the intended temperature rise uniformity is unacceptably limited in spite of a considerably complicated mechanism required for this purpose.
The apparatus well known from the above-mentioned Japanese Disclosure Gazette No. 1973-59976 is claimed to minimize the variable temperature rise within an ampule by irradiating the lower portion of the ampule (held upright) with the microwaves. However, this apparatus also has various problems remaining unsolved due to the simple manner of microwave irradiation. Namely, a low heating efficiency requires the correspondingly long heating furnace as well as a high microwave capacity and makes it impossible to achieve an efficient sterilization.
Moreover, the microwave irradiation performed in such manner causes substantially only the liquid phase of the medical fluid within the ampule to be heated and the unfilled space within the same ampule remains at a lower temperature even after medical fluid has attained a temperature high enough to be sterilized. In other words, germs on the inner wall of said unfilled space are left unsterilized.
Finally, the above-mentioned Japanese Disclosure Gazette No. 1979-34590 proposes a countermeasure of inverting the ampule. However, the mechanism for this purpose is considerably complicated and it is practically difficult for such mechanism to realize continuous processes of sterilization.
Additionally, if conditions of the microwave irradiation are changed according to the type of medical fluid, the speed of the sterilizing process will necessarily vary and accordingly the speed of the medical fluid filling process must be changed. This often results in a repression of production rate.